The invention relates generally to focusing co-axial laser beams of different wavelengths, and in particular to varying of the size of a focused laser spot of one color while not affecting the size of a second, co-axial focused laser spot of a second color. More specifically, this invention relates to the adjustment of laser spot sizes in a three-color digital laser scanning printer for use in printing lenticular images.
Lenticular overlays are a means of giving images the appearance of depth or motion. A lenticular image is created using a transparent upper layer having narrow, parallel lenticules, semi-cylindrical lenses, on an outer surface, and an image-containing media. The two layers form a lenticular system wherein different portions of an image are selectively visible as a function of the angle from which the system is viewed.
If the image is a composite picture made by bringing together a number of different parts of a scene photographed from different angles and the lenticules are oriented vertically, each eye of a viewer will see different elements and the viewer will interpret the net result as depth of field. The viewer may also move her head with respect to the image thereby observing other views with each eye and enhancing the sense of depth. When the lenticules are oriented horizontally, each eye receives the same image. In this case, the multiple images give illusion of motion when the composite image is rotated about a line parallel to a line formed by the viewers eyes.
One method of creating these images uses a lenticular sheet with a color photographic emulsion on the side opposite the lenticules. The stereoscopic images are exposed onto the lenticular material by a laser scanner and the material is processed to produce the lenticular image. See for example, U.S. Pat. No. 5,697,006 issued Dec. 9, 1997 to Taguchi et al.
The color image exposed on the lenticular material is produced by three lasers each, a different color, e.g. red, green, and blue. Typically, the red laser exposes a cyan layer of the emulsion, the green laser exposes a magenta layer, and the blue laser exposes a yellow layer. It is important that the width of each scanned line be the correct size. If exposure by one color produces a line which is wider than by exposure of another color, a colored xe2x80x9cfringexe2x80x9d will be produced around each scan line. This will result in a colored shadow visible in the resultant lenticular image. This color fringing may be visible in non-lenticular, conventional laser printers, however, the magnifying effects of the lenticules make a lenticular printer more sensitive to line width error.
The widths of the lines are a function of the intensity distribution of the focused laser spot and of the emulsion characteristics. The emulsion characteristics are generally different for each color. Thus, an identical intensity distribution of separate laser wavelengths will not necessarily produce identical linewidths. It thus becomes important to have good control over the intensity distribution and thus, spot size produced by each laser beam.
Since the intensity distribution of a point focus is not constant or uniform, it is typical in the art to define a xe2x80x9cspot sizexe2x80x9d as it relates to the intensity distribution. For example, when a gaussian laser beam is focused, the intensity distribution of the focused spot is a gaussian distribution with a spot size at the 1/e2 diameter equal to             .635      ⁢              xe2x80x83            xc3x97              xe2x80x83            ⁢      λ        NA    ,
where NA equals Numerical Aperture. When a uniform intensity distribution is focused, the result is an airy disc whose central diameter is             1.22      ⁢              xe2x80x83            ⁢      λ        NA    .
So, by truncating an incident gaussian beam, it is possible to slightly change the intensity distribution of the focused slot and thus the spot size.
It is possible to control each laser""s spot size separately if the laser beams are spatially separated. However, there are situations in which the laser beams are combined co-axially. It is not practical or convenient to separate the laser beams, once combined, in most applications. For example, the beams may be carried by separate fiber optic cables and then combined through a fiber multiplexer into a single fiber optic cable. The beams may also be combined co-axially by dichroic prisms.
The intensity distribution of the focused laser spot is a function of the laser wavelength, the aberrations of the optical system focusing the laser beam, and the intensity distribution of the laser inside the optical system. For a given optical system of fixed image quality, i.e., the aberrations are constant, it is possible to change the intensity distribution of a focused spot by changing either the wavelength of the light or by changing the intensity distribution of the laser somewhere in the optical system.
It is an object of this invention to provide a method and apparatus for changing the spot size of a focused laser spot of one color while not substantially changing the spot size of a co-axially focused laser spot of a second color. It is another object of the invention to provide a means for continuous adjustment of the spot size of a focused laser spot of one color while not substantially changing the spot size of a co-axially focused laser spot of a second color.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a method of adjusting a first spot size for a first color component of a multiple color co-axial laser beam comprises the steps of focusing the multiple color co-axial laser beam; filtering the multiple color co-axial laser beam with a filter to adjust the first spot size. The filter is opaque to the first color component in an annular region and transparent to the first color component in a center region of the multiple color co-axial laser beam. The filter is transparent to a second color component of the multiple color co-axial laser beam.
According to another aspect of the present invention, a lenticular image is formed on a lenticular sheet having a photographic emulsion coated on a side opposite the lenticules. A beam used to form the image is comprised of at least two intensity modulated beams of light of having different wavelengths, and focused spots from the beam are scanned on the lenticular material. The spots are scanned in a direction parallel to the long axes of the cylindrical lenses to form a latent lenticular image in the photographic emulsion. A filter is placed in the path of the co-axial beams and is opaque in an annular region and transparent in the center region to a first laser wavelength and transparent to a second laser wavelength. The filter alters the incident intensity distribution to one set of laser wavelengths while not affecting the incident intensity distribution of another set of laser wavelengths. The result of this is a final focused laser spot size which is altered with respect to one wavelength.
In an alternate embodiment, the filter is placed in the path of the co-axial beams in an area where the beams are either converging or diverging, i.e., not in collimated light space. By translating the filter along a line parallel to the co-axial beam direction, the filter can be adjusted to apodize more or less of one set of laser wavelengths. This has the effect of continuously varying the focused intensity distribution over a predetermined range.
The invention provides an accurate method and apparatus for varying the intensity distribution, and thus the spot size, of a focused laser of one wavelength while not affecting the intensity distribution, and thus spot size, of a co-axially focused laser of a different wavelength.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.